Heat exchangers are intended to make it possible to obtain a heat transfer between two media flowing separately from each other in a particularly effective manner, that is to say they are intended to transfer as much heat as possible with the least possible exchange area. At the same time, they are intended to offer only little resistance to the substance flows, in order that least possible energy has to be expended for operating the pumps used for delivery. If highly aggressive or corrosive media are passed through the heat exchanger, possibly even at elevated temperatures of over 200° C., all the materials in a heat exchanger that are in contact with the medium must be adequately resistant to corrosion. This includes not only the exchange areas but also all the seals and bushings. Furthermore, the structure of heat exchangers should be made such that, if necessary, complete emptying of the heat exchanger is easily possible, for example for maintenance work.
Plate heat exchangers are a special form of heat exchangers. They are distinguished by a particularly compact design. The plates of a plate heat exchanger generally have in the region of the exchange area an embossed or grooved structure, often also referred to as a herringbone pattern or chevron pattern. The embossing imparts strong turbulence to the medium flowing in the gap between two neighboring plates, which is conducive to the heat transfer. At the same time, such a structure offers relatively little flow resistance to the medium. This is largely in keeping with effective heat transfer with least possible pressure loss.
The plates usually rest loosely on one another at the edges and are separated by seals. Since plastic seals can only be used at temperatures no higher than 300° C., in the case of heat exchangers with plates made from metallic materials, for higher operating temperatures or pressures, the plates are brazed or welded to one another at the edge.
The gap between two neighboring plates respectively forms a sealed chamber. Along with the embossing of the plates, the volume of the chambers is a crucial factor in determining pressure loss and efficiency in the heat transfer. A large chamber volume is conducive to both and therefore desirable. However, this is also at the expense of an operational risk. If no supporting segments are used in the chambers, the unforeseen buildup of a great difference in pressure between neighboring chambers may cause strong deformation of the metal plates or, in the case of brittle materials, easily result in plate rupture. Heat exchanger plates of this form are produced from metallic materials, in particular from corrosion-resistant steels, titanium or tantalum. Graphite is also commercially used.
Sintered SiC ceramic (SSiC) is a universally corrosion-resistant, but brittle material, which is free from metallic silicon, by contrast with silicon-infiltrated silicon carbide (SiSiC) SSiC is ideally suited as a material for the exchange area of heat exchangers on account of its very high thermal conductivity. Moreover, SSiC can also be used at high temperatures up to far above 1000° C. By contrast with SiSiC, SSiC is also resistant to corrosion in hot water or strongly basic media.
In spite of its fundamentally good suitability for heat exchangers, sintered SiC ceramic (SSiC) is currently still not commercially used in plate heat exchangers, but if at all in shell-and-tube heat exchangers. The reason for this is that so far there has been no available design and no available production process that are appropriate for ceramic and make it possible to produce plate heat exchanger components from SSiC for apparatuses with adequate heat transfer performance and the required low pressure loss.